Corrosion-resistant acrylic coatings

ABSTRACT

This invention relates to a corrosion-resistant coating capable of being  lied to various substrates, e.g., particularly metal and plastic surfaces, as a single coat characterized as having a high-gloss, good adhesion and a high degree of flexibility. The corrosion inhibiting composition comprises an acrylic resin containing an effective amount of a corrosion-inhibiting pigment system consisting essentially of critical amounts of at least one zinc phosphate, zinc molybdate and at least one zinc salt of a benzoic acid.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the U.S. of America for governmental purposes withoutthe payment of any royalties thereon or therefor.

CONTINUATION APPLICATIONS

This application is a Continuation-In-Part of copending application Ser.No. 07/627,670 filed Dec. 14, 1990 which in turn is aContinuation-In-Part of copending application Ser. No. 07/593,416 filedSept. 28, 1990 now U.S. Pat. No. 5,059,640 which in turn is aContinuation-In-Part of copending application Ser. No. 07/442,085 filedon Nov. 28, 1989 now U.S. Pat. No. 5,043,373 which in turn is aContinuation-In-Part of copending application Ser. No. 07/331,200 filedMar. 28, 1989 now U.S. Pat. No. 4,885,324 which in turn is aContinuation of application 07/211,026 filed June 16, 1988, nowabandoned.

BACKGROUND OF THE INVENTION

This invention relates to coating compositions and more specifically tocorrosion-resisting acrylic coating compositions which can be applied byvarious methods directly onto surfaces and particularly metal surfacesand polymeric or plastic substrates without the need for an undercoat.

Metal substrates, and particularly metal exposed to extremeenvironmental conditions, require the protection of coatings capable ofresisting corrosion e.g., ships and military aircraft exposed toseawater spray and other corrosive reagents including acid-forminggases, e.g. sulfur dioxide and the like. In addition to ships andaircraft, particularly military aircraft, various types of machinery andfarm equipment used in industrial environments where fossil fuelsgenerate corrosive agents need protection against various reagents. Inaddition to corrosion, it is important that the coatings have physicalcharacteristics which enable the coating to be applied to the substratewithout difficulty. These coatings should also exhibit good adhesion andhave a high degree of flexibility.

Presently, coatings attempting to comply with the above-mentionedrequirements rely on the use of a plurality of films, i.e. an undercoatwith a topcoat, comprising, for example, an epoxy undercoat and apolyurethane topcoat. The epoxy primers used in the military arespecifically designed to adhere to metal surfaces. Many of theprimecoats, however, generally require a topcoat, since the primers donot have the required flexibility, particularly at low temperatures, andtherefore results in extensive cracking and/or blistering in areas whichare highly flexible. Moreover, the primer coats are not generallyresistant to harsh weather conditions and are difficult to formulate inthe multi-colors required for military aircraft. The acrylic coatingcompositions of this invention are resistant to harsh weather conditionsand various chemicals including saltwater and have the required degreeof flexibility. In addition, it was necessary heretofore to provide aplurality of films of the coatings to form a total film thicknessranging up to about 0.005 inches e.g. up to 10 mils or greater whichadds considerable weight. Thus, the multi-coat finishes require aplurality of films which are very time consuming in applying because ofthe drying time between each application. Moreover, it is obvious thatthe removal of the two coat system can likewise be difficult and timeconsuming and requires the use of large amounts of organic solventscausing objectionable emissions.

In accordance with this invention, the corrosion resistant costingscomprise an acrylic binder, i.e. an acrylic resin such as AcryloidAU-608S or 608X. These particular acrylic resins are acrylic polyolsdesigned to produce hard, resistant, desirable coatings when reactedwith isocyanates. More specifically, the coating compositions of thisinvention comprises approximately 100 parts by weight of an acrylicresin in combination with about 0 to 1000 parts by weight of at leastone organic paint solvent for said resin and from about 0 to 140 partsby weight of a TiO₂ pigment, e.g. titanium dioxide pigment in the formof vesiculated beads or combinations of TiO₂ beads and pigment and fromabout 0.01 to 300 parts by weight of a combination ofcorrosion-inhibiting pigments consisting essentially of about 10 to 120parts by weight of at least one zinc phosphate, 40 to 260 parts byweight of zinc molybdate, and 1 to 30 parts by weight of at least onezinc salt of a benzoic acid, e.g. substituted zinc benzoate. Forpurposes of this invention, all three of the zinc salts, as disclosedherein, are essential in their relative proportions to provide thecorrosion resistance required for coatings. Other pigments, andparticularly, spherical TiO₂ particles and the vesiculated beads e.g.TiO₂ beads may be used in combination with the three zinc salts asdisclosed herein.

The coating composition of this invention maybe applied as a single coatdirectly onto a hard surface such as metal, plastic or polymericsurfaces and do not require a top coat to provide a high gloss,corrosion resistant film. It is generally known that low gloss coatingsare appropriate for camouflage purposes particularly on most of theouter exposed surfaces of military aircraft and the like. On the otherhand, low gloss coatings are not appropriate for the internal orunexposed surfaces such as the areas around engine inlets, ducts,landing gears, etc. Moreover, aircraft other than the military aircraft,require high gloss and high visibility coatings. It was thereforebelieved that in order to obtain a coating which would exhibitoutstanding corrosion resistance, the amount of pigment, i.e. (PVC)pigment volume concentration had to be relatively high which wouldtherefore result in a low gloss finish. It was believed that it was notpossible to obtain a final coat which also had high gloss and goodcorrosion resistance at high pigment volume concentrations.

SUMMARY OF THE INVENTION

This invention relates to a corrosion-resistant coating capable of beingapplied to various substrates as a single coat having high-gloss, goodadhesion and a high degree of flexibility. The corrosion inhibitingcoating comprises an acrylic resin and a corrosion inhibiting pigmentsystem consisting essentially of critical amounts of zinc phosphate,zinc molybdate and at least one zinc salt of a benzoic acid.

It is an object of this invention to provide a glossy,corrosion-resistant coating, which can be applied directly onto asurface without the need for an undercoat.

It is another object of this invention to provide a coating which isresistant to corrosion and various other chemicals, is flexible,resistant to different weather conditions and has good adhesion to metalsubstrates.

It is still a further object of this invention to provide a corrosionresistant coating capable of reducing the time, the manpower and thematerials normally required for applying said coating on varioussubstrates.

It is still a further object of this invention to provide a one-coatsystem useful for both military and civilian aircraft of substantiallyreduced thickness which reduces the weight added to the aircraft whileat the same time providing the necessary corrosion resistance.

These and other objects of the invention are accomplished by providingan acrylic resin containing an unique combination of pigments which canbe applied on a variety of substrates.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a relatively high-gloss, corrosion resistantcoating composition which functions as the primary coat or topcoat, hasgood adhesion characteristics, is highly flexible and resistant tochemical and corrosive environments. More specifically, this inventionis directed to a relatively high-gloss, corrosion-resistant coatingwhich comprises for each 100 parts by weight of an acrylic resin, e.g.,an acrylic-urethane resin from about 0 to 1000 parts by weight of atleast one organic paint solvent and preferably from about 50 to 500parts by weight of solvent including xylene, toluene, mineral spiritsand combinations thereof, and from about 0 to 140 parts by weight ofTiO₂ pigment e.g., preferably from about 40 to 100 parts by weight ofTiO₂ pigment. The TiO₂ pigment may consist of from about 0 to 100 percent by weight of the total amount of TiO₂ of vesiculated beads.

In addition to the above, the coating composition must contain fromabout 0.1 to 300 parts by weight and preferably from about 20 to 100parts by weight of a combination of corrosion inhibiting pigmentsconsisting essentially of about 10 to 100 parts by weight of at leastone zinc phosphate, 40 to 260 parts by weight of zinc molybdate, and 1to 30 parts by weight of at least one zinc salt of a benzoic acid, e.g.zinc benzoate and/or a zinc salt of a substituted benzoic acid whereinthe substituants include NO₂ and/or hydroxy radicals.

The acrylic polymers, useful for purposes of this invention, includecopolymers, and terpolymers of methacrylic and acrylic acid whichcontain up to about 15 weight percent of carboxyl functionality. Forexample, polymers derived from acrylates, e.g. methyl methacrylate andacrylic or methacrylic acid may have molecular weights ranging up toabout 500,000. The acrylic resins may be provided in solution atconcentrations ranging up to 60% by weight, and therefore it may not benecessary to add additional solvent. However, various solvents may beused in preparing the coating including xylene, toluene, mineralterpene, methyl ethyl ketone, methyl isobutyl ketone, ethyl cellosolve,butyl cellosolve, cellosolve acetate, ethyl acetate, butyl acetate,methyl isobutyl carbinol, isopropanol, n-butanol, cyclohexanone ormixtures thereof in any proportion. Usually, the content of thenon-volatile components in the coating composition of this inventionranges up to about 90% e.g., 25-50% by weight of the total composition.The amount of solvent may vary depending on the viscosity and method ofapplication of the coating.

The pigment system of this invention is unique in that it consistsessentially of at least one zinc phosphate, e.g. zinc-barium phosphate,a zinc salt of benzoic acid or a substituted benzoic acid and zincmolybdate. These three zinc pigments alone or in combination with otherknown pigments, e.g. TiO₂, provide a coating having outstandingcorrosion inhibiting characteristics which enables a single film of thecoating to be used as a primer or as the topcoat.

Of the various zinc salts of benzoic acids, it was found that thepreferred zinc benzoates include the benzoic acid salts having at leastone substituent, i.e. the hydroxol and/or the nitro (NO₂) substituant.The preferred zinc phosphates, e.g. zinc-barium phosphate, arecommercially available as Phos-Plus (J0866) from the Minerals PigmentCorporation. The zinc molybdates are well known zinc compoundscommercially available as Moly-White. In addition to the zinc saltspigment system as disclosed herein, other known pigments, particularlytitanium dioxide, zinc oxide and the like may be used in the coating toprovide reinforcing characteristics and to add color, opacity and hidingpower to the coating. In addition, other additives well known in thecoating art such as color or tinting agents may be added to the binderin small but effective amounts and include such compounds as antimonyoxides, barium sulfate, calcium carbonates and one or more of theorganic pigments such as the phthalocyanine colors, e.g. greens orblues, etc.

It was unexpected that the combination of zinc molybdate, the zinc saltsof benzoic acid and the zinc phosphates synergistically improved thecorrosion resistance of the coating even at low concentrations. It wasfound that the specific combination of the zinc molybdate, zinc salts ofbenzoic acid and zinc phosphates in the relative ratio's stated hereinimproved the corrosion-resistance substantially when compared to the useof either one of these zinc salts alone. Thus, by decreasing the pigmentvolume concentration (PVC) of the pigment system in the acrylic binder,a coating of higher gloss can be obtained without impairing thecorrosion resistance.

In general, the coatings are prepared by mixing all of the ingredientsinto the acrylic binder and applying the film-forming composition to thesubstrate at thicknesses ranging from about 0.001 to 0.005 inches or upto about 20 mils and preferably from about 1 to 10 mils. A dispersion orsolution of the coating may be accomplished by conventional mixingmethods including the use of agitation with a mixer, ball mills, etc.The application of the coating onto the substrate e.g. metal surface,may be made by known coating procedures such as spraying, dipping,brushing, roller coating, etc. The viscosity of the coatings for theparticular application may be adjusted by the addition of one or moreknown organic solvents within the numerical ranges disclosed herein.After the coating is applied to the surface, the solvent is allowed toevaporate at room or elevated temperatures and the film cures to acoating having the desired characteristics.

The particular zinc phosphate used in preparing the coating compositionhas an average particle size of about 5.0 microns and may becharacterized as a zinc-barium phosphate. The zinc salts of benzoic acidare specifically characterized as having at least one hydroxyl groupand/or nitro (NO₂) substituent with a molecular weight of about 100-500,a density of about 2-3 grams per mililiter and a particle specificsurface area of 16m² /gram. The benzoic acid salts are commercialproducts obtained from BASF and identified as Sicorin-RZ. The zincmolybdate has a particle size of about 4.0 microns and is commerciallyavailable as Moly-White 101. The titanium dioxide is preferablyspherical with a particle size of about 0.25 microns. The term zincphosphates includes the mixed zinc salts and particularly thezinc-barium phosphates, zinc-aluminum phosphates and variouscombinations of zinc phosphate with mixed zinc phosphate salts in anyproportion.

The coating composition may optionally contain other additives such asan ultraviolet light stabilizer, antioxidants or both. The ultravioletlight stabilizer can be present in amounts of 1-10% by weight, based onthe weight of the binder; the antioxidant can be present in amounts of0.1-3% by weight, based on the weight of the binder. Typical ultravioletlight stabilizers are benzophenones, triatriazoles, triazines,substituted benzenes, organophosphorous sulfides, and substitutednitriles. The coating composition of this invention may also containother known materials, such as driers, antioxidants, fungicides, etc. inamounts for their intended function with various solvents for suchmaterials. Organic salts (e.g., an octoate or naphthenate) of metals(e.g., cobalt, calcium, zirconium, manganese, bismuth or antimony) areavailable from Nuodex Corporation under the name "Nuxtra". The coatingcomposition may also contain fillers which may or may not havepigmentary properties. These fillers are exemplified by talc, silica,barium sulfate, calcium sulfate, calcium carbonate, calcium silicate,iron oxides, mica, aluminum silicate, clay and mixtures thereof.

The coating composition can be applied to a variety of substrates by anyof the conventional methods. Substrates include, for example, metal,wood, glass, or plastics such as polypropylene, polystyrene, and thelike. The coating is suited also for application over pretreated orunprimed metal.

The hydrocarbon solvents useful for purposes of this invention includesa mixture of solvents, e.g. mixtures of one or more paint solvents suchas benzene, toluene, xylene, and aromatic naphtha. Other solventsinclude the ester solvents such as ethyl acetate, butyl acetate,cellosolve, hexyl acetate, amyl acetate, ethyl proprionate, and butylproprionate. Ketone solvents include acetone, methyl ethyl ketone,methyl isopropyl ketone, methyl isobutyl ketone, diethyl ketone, andcyclohexanone. Glycol ester solvents include ethylene glycol monoethylether acetate, diethylene glycol monoethyl ether acetate, etc.

The coating has outstanding performance when exposed to high intensityof light, extreme cold conditions, hot lubricating oils and otherchemicals normally found in aircraft operations. By utilizing thecoating composition of this invention, a high gloss corrosion resistantfilm can be obtained on various substrates as the top coat. The coatinghas properties which function as a primer and more important as a topcoat highly adherent, flexible, chemical resistant and resistant to allweather conditions. The coatings of this invention lowers the risk offailure due to cracking especially at low temperatures and are easilytouched-up since only one coating need be applied. Since one coat issufficient, it needs less time for application and removal which saveson manpower that would generally be needed in the preparation of a highgloss two coat system. Moreover, the present coating provides protectionat lower film thicknesses thereby reducing the weight of the coatingcompared to a two coat paint system which is important for aircraftcoatings.

The following illustrate the coating compositions of this invention.

    ______________________________________                                        EXAMPLE I                                                                     ACRYLIC RESIN                                                                 Components        Parts by Weight                                             ______________________________________                                        Acrylic resin (60% solids)                                                                      55.0                                                        (AU-608S)                                                                     TiO.sub.2 (50% VSBD)                                                                            28.9                                                        Zinc Phosphate    2.9                                                         Zinc Benzoate     0.3                                                         Zinc Molybdate    5.9                                                         Organic Solvents  7.0                                                                           100.0                                                       ______________________________________                                    

The above composition has the following characteristics:

    ______________________________________                                        PVC =                 0.229                                                   Wet Density (g/ml) =  1.44                                                    Weight % Solids =     0.709                                                   VOC (g/l) =          419.93                                                   ______________________________________                                    

The degree of anti-corrosion performance of the coating can be measuredby ASTM test D610-68 entitled "Evaluating Degree of Rusting on PaintedSteel Surfaces", ASTM D1654-79a entitled "Evaluation of Painted orCoated Specimens Subjected to Corrosive Environments", and ASTM D714-56entitled "Evaluating Degree of Blistering of Paints". The coating can betested further in ASTM B117-73, entitled "Method of Salt Spray (Fog)Testing" wherein the composition is applied onto steel panels which arescribed and subjected to salt-fog spray. Scribing is achieved byscratching an "X" in the coating through to bare steel using a cuttingtool. The amount of rusting at the scribe is assessed on a scale of 0-10where 10 is no corrosion and 0 is complete failure. Ratings of 5 andabove are acceptable for anti-corrosive compositions tested inaccordance with ASTM D-1654-79.

Scribe creepage or underfilm corrosion is determined in accordance withASTM D1654-79a on a scale of 0-10 where 10 is no corrosion and 0 is 5/8inch or more creepage from the scribe. Ratings of 3 or above areacceptable for anti-corrosive compositions. Blistering in a coating isdetermined in accordance with ASTM D714-56. This method describesblister size as numbers 2, 4, 6, 8, and 10, where 2 is a large blister1/4 inch or larger in diameter, 8 is a small blister less than 1/16 inchin diameter and 10 is the absence of blistering.

It is obvious that there are other variations and modifications whichcan be made with respect to this invention without departing from thespirit and scope of the invention as particularly set forth in theappendant claims.

The invention claimed:
 1. A coating composition comprising about 100parts by weight of an acrylic resin from about 0 to 1000 parts by weightof at least one organic solvent, from about 0-140 parts by weight of aTiO₂ pigment, and from about 0.01 to 300 parts by weight of acombination of corrosion-inhibiting pigments consisting essentially ofabout:(a) 10-120 parts by weight of a zinc phosphate, (b) 40-260 partsby weight of zinc molybdate, and (c) 1-30 parts by weight of at leastone zinc salt of a benzoic acid.
 2. The coating composition of claim 1further characterized in that the salt is a substituted zinc benzoate.3. The coating composition of claim 1 further characterized in that thezinc phosphate is a zinc-barium phosphate.
 4. The coating composition ofclaim 1 further characterized in that from 0 to 100% by weight of thetotal amount of the TiO₂ pigment in the coating is in the form ofvesiculated beads.
 5. The coating composition of claim 1 furthercharacterized in that from 10 to about 50% by weight of the total amountof the TiO₂ pigment in the coating is in the form of vesiculated beads.6. The coating composition of claim 4 further characterized in that theTiO₂ pigment is present in the coating in an amount ranging from about40-100 parts by weight.
 7. The coating composition of claim 1 furthercharacterized in that the combination of corrosion-inhibiting pigmentsis present in an amount ranging from about 20 to 100 parts by weight. 8.The coating composition of claim 1 further characterized in that theacrylic resin is an acrylic-urethane resin and the combination ofcorrosion-inhibiting pigments consist essentially of about:(a) 30-100parts by weight of a zinc-barium phosphate, (b) 60-210 parts by weightof a zinc molybdate, and (c) 3-25 parts by weight of a zinc salt of abenzoic acid.
 9. The coating composition of claim 8 furthercharacterized in that the zinc salt of the benzoic acid is a substitutedbenzoic acid.
 10. The coating composition of claim 9 furthercharacterized in that the substituted benzoic acid is a hydroxy andNO₂ - substituted benzoic acid.
 11. The coating composition of claim 9further characterized in that the substituted benzoic acid is a NO₂-substituted benzoic acid.
 12. A method of preparing acorrosion-inhibiting coating which comprises adding to an acrylic resinfrom about 0.01 to 300 parts by weight for every 100 parts by weight ofresin of a combination of pigment consisting essentially of from about10 to 120 parts by weight of a zinc phosphate, 40 to 260 parts by weightof zinc molybdate and 1 to 30 parts by weight of at least one zinc saltof benzoic acid.
 13. The method of claim 12 further characterized inthat from about 50 to 500 parts by weight of at least one organicsolvent is present for each 100 parts by weight of said acrylic resin.14. The method of claim 13 further characterized in that the coatingcontains from about 0 to 140 parts by weight of TiO₂ pigment.
 15. Themethod of claim 14 further characterized in that the zinc phosphate is azinc-barium phosphate and 10 to 50 percent by weight of the total amountof TiO₂ pigment is in the form of vesiculated beads.